1. Field of the Invention:
The present invention relates to a radiographic diagnostic apparatus which employs a radiographic sensor array.
2. Description of Related Art:
Radiographic images may be electronically produced without using X-ray films by using either a combination of an image intensifier (a fluoroscopic intensifier) and a camera tube or a linear X-ray sensor array. The radiographic technique that utilizes a linear X-ray sensor array employs an X-ray fan beam which is obtained by passing a primary form of radiation through a slit. This can remove scattered radiation from an object and in that way provide good-quality images. Further, it can be equipped with a screen which is larger than the type employed with image intensifiers. Thus, an increasing future demand therefor is anticipated.
Radiographic techniques that involve driving of a linear X-ray sensor array include one techniqus in which, while an X-ray tube is being rotated around a focal spot thereof, a line slit and the linear X-ray sensor array are moved in parallel with each other or rotated together in synchronization with the rotation of the X-ray tube so as to scan the object and produce an image thereof. This is an application of a technique called moving-slot radiography that employs films, and is described on page 85 in the publication Applied Radiology, the issue dated November-December 1977, and on page 783 in the publication Radiology, Vol. 128, 1978. In these publication the method of moving-slot radiographic is described, in which an object is irradiated with the X-ray fan beam, and a radiographic image thereof is produced by scanning the object by the linear sensor array which is moved in synchronization with the sweep of the fan beam without using any photographic film.
FIG. 8 shows the principle of the above-described known type of radiography. X-rays irradiated from a focal spot 2 of an X-ray tube 1 are shaped into a fan beam by a line slit 3, and the fan beam is detected by a linear X-ray sensor array. While the X-ray tube 1 is being rotated, the line slit 3 and the linear X-ray sensor array 4 are moved in the direction indicated by the arrows in synchronization with the rotation, of the X-ray tube 1 so as to produce a radiographic image of the object interposed between the line slit 3 and the linear X-ray sensor array 4.
The output capacity of the X-ray tube required for the above-described radiography will be considered. Assuming that the surface to be scanned by the sensor array is divided into 1000 lines, then a time which is 1000 times as long as that required to produce an image on a film surface is necessary. For example, if the time for scanning a single line is loomsec, then the time required to produce an image on a flat film surface is 100 msec.times.1000, i.e., 10 sec. Conversely, in order to scan 1000 lines in the same period of time, an X-ray output which is 1000 times as high as that required for conventional radiography is necessary. Thus, radiography which employs a linear X-ray sensor array requires an X-ray tube of excessively large capacity.
In order to obviate the above-described problem, the X-ray tube and the linear sensor array could be brought closer to each other so as to increase the dose of X-ray radiation. However, this will cause the following disadvantages. FIGS. 9a and 9b illustrate the principle involved when the X-ray tube is brought closer to the sensor. The two views are respectively taken from the side and the end of the linear sensor array 4. When the X-ray tube 1 and the focal spot 2 are respectively moved to the positions indicated by 1' and 2', the angle formed when the surface to be scanned is viewed from the focal spot 2 increases to the angle formed when the surface to be scanned is viewed from the focal spot 2', as shown in the drawings. As a result, the magnification factor and hence the degree of distortion of the image of the object 7 so produced increases in the vicinity of the ends of the screen, making it difficult to extract diagnostic information from the image produced.